The invention relates to a device as well as to a method for the partial processing, e.g., galvaniz galvanizing, electrochemical pickling or electrochemical cleaning of bar-shaped objects in dip plants, namely in galvanizing lines, pickle lines and cleaning lines.
In the following description, the objects to be processed will be referred to as the rod. Devices for electrochemical pickling, galvanizing and electrochemical cleaning of long stretched-out cylinders, in particular of round rods, are well known. At one end or at both, these rods may be tapered and/or threaded. One such example is the cylinder in shock absorbers in vehicles. The invention however is not limited to the processing of round rods. It is also suited for the treatment of rods having another section.
The electrochemical processing of the rods serves for example to improve the wear and corrosion resistance properties of shock absorber cylinders. To this purpose, the rods are electrolytically plated with hard-chromium at only such surfaces which are subjected to load during operation. The other places of the rods are intended to remain uncoated or to be partially provided with a thin layer of flash chrome for temporarily protecting their surface. In order to improve the adherence of the chromium layer, the surface is electrochemically pickled first. Insoluble electrodes are preferably employed for either step in the process. Between the various steps of the treatment, the rods are cleaned in galvanizing lines. A conveying facility brings the rods from one processing station to the other.
To partially process the rods in their central region, the ends have to be masked, i.e., shielded in such a way that no metal or, as a temporary protection flash chromium, can be deposited onto these dimensionally accurately beforehand defined areas. The boundaries between the area of deposition and the areas not to be coated generally have close tolerances. On certain shock absorber cylinders, the uncoated area must verge into the coated area e.g. within a range of xc2x11 millimeter only. The thickness of the coating to be deposited must be uniform up to this boundary. Edge effects, i.e., an increase or decrease of the coating""s thickness on its boundaries must be avoided. The goal of such an accurate coating is to eliminate the need for subsequent polishing.
In the plants of the art, the rods are attached to racks where they are processed, the proper racks being attached to movable flight bars. The racks are provided with individual masks in such a manner that the two ends of the rod are not coated in the predetermined region. In an electroplating plant, masks and racks must be available in sufficient number for all the current rod lengths, diameters and areas of plating. Since the racks also serve to feed the current conducted by the bath to the rods, they have to be made of a conductive metal. Prior to galvanizing, the metal has to be protected by layers of plastic.
The document DE-AS 11 03 103 describes a device for galvanic chromium plating of the outer surface of disk valve stems. In order to permit partial galvanizing of the stems, said stems are accommodated in a centric manner in tubular anodes and are fastened by the upper side in a stepped metallic core. Current is fed via the metallic core, the metallic core being separated from the electrolytic cell by an insulating body. The metallic core and the insulating body constitute the boundary for the galvanic coating at the upper end of the stem. The lower end is protected against undesired galvanizing by a protective lacquer.
The disadvantage of this embodiment is that each body to be galvanized has to be attached individually and that the protective lacquer has to be applied with dimensional accuracy to one end of the body. To remove the protective lacquer once galvanizing has been completed is also complicated.
DE 197 22 983 A1 indicates a method and a device permitting to partially electrochemically process, in particular to electrochemically process rods of various dimension in dip plants.
For each rod, defined adjustable masks with sealing means in the form of terminal collars are employed at either end of the rod to delimit the surfaces to be treated. In a charging station, several electrically conductive grippers, which are attached to the flight bar, simultaneously grasp one rod each on one side. By immersing the rods into the electrolytic processing station, each rod enters an individual cell constituted by the rod and one stationary tubular electrode. Either of the electrolytically effective upper and lower end of the electrode is defined by a tubular, axially adjustable mask. Each end of the mask is terminated by a collar. By immersing the flight bar, the rod first passes through the upper collar, enters the electrode and eventually traverses the lower collar. By displacing the upper and lower mask independently from one another parallel to the axis of the rod, the two plating boundaries, or the surface on the rod which is to be plated, are uniformly adjusted for each flight bar or for each row of rods on the flight bar. This method overcomes the drawbacks of the prior art rack technique, in which for each rod dimension a special mask in the form of caps or holders is needed. In practical operation however, the objects to be processed are often hanging crookedly since metal has deposited on the tongs holders or since the holders are damaged. When the no longer centrically hanging objects to be processed are being entered into the masks, the collars laterally gape and may be damaged by sharp edges. As a result of the damaged collars, accurately dimensioned galvanizing is no longer possible. What makes it even more difficult is that damages on the collars only become noticeable once the galvanized articles have exited the plant, since the collars, when immersed in the electrolyte, cannot be seen from the surface of the bath. The articles processed under these conditions are unserviceable.
The basic problem of the present invention is therefore to avoid the drawbacks of the methods and devices of the art for shielding rods during the galvanizing process and in particular to keep the amount of work required for shielding such areas on the rods that are not to be galvanized as small as possible. Moreover, the method is intended to work perfectly, even under manufacturing conditions.
The solution of this problem is given by a self-centering device according to the instant invention.
The device according to the invention serves for the partial electrochemical processing of rod-shaped objects in dip plants. The device comprises
a. at least one plating tank,
b. in the at least one plating tank, tubular electrodes and at least one tubular membrane carrier in which the objects to be processed may at least partially be entered, and
c. membrane holders arranged within the at least one membrane carrier.
The tubular membrane carriers are preferably made of a chemically stable material which is electrically nonconductive on its surface at least.
According to the dimensions of the objects to be processed, the device is provided with axially adjustable, electrically nonconductive shielding facilities. At least one shielding facility is provided for each rod. A shielding facility may be provided for example at the lower end of the rod within the plating tank in order to prevent this portion of the rod from being electrochemically treated. In this case, the upper portion of the rod could remain untreated by not submerging it into the solution. Shielding facilities may also be provided at either end of the rod, though.
The shielding facilities comprise at least one cage (34) within the tubular membrane carrier. The cages are arranged in such a manner that the objects to be processed are capable of being pushed through them and are each formed by at least one cage cover and at least one cage bottom, respectively. A membrane holder holding a membrane is carried in a radially movable fashion between the cage cover and the cage bottom in each cage. At least one inner centering spring for guiding the objects to be processed and at least one outer centering spring for centering the membrane holder and the membrane in the cage are provided on each membrane holder. The membranes are preferably made of a chemically stable, extensible and electrically nonconductive material.
The cages are arranged on that side of the shielding facility which is facing the center of the rod.
The outer centering springs are preferably rod-shaped and designed as leaf springs and are tangentially fastened to the outer surfaces of the membrane holder. Their function is to center the membrane holder and the membrane in the cage and they oppose a radial pressure exerted by an off-center rod onto the membrane and the membrane holder. The force of the outer centering springs is adapted to the tensile property of the elastic membranes. It should be selected to be so large as to prevent the membranes from gaping as a result of lateral pressure applied by the objects to be processed or, in the extreme, from tearing, thus destroying the shielding effect at this place.
The force of the inner centering springs should however be selected to be smaller than the spring force of the holding tongs for the rods in order not to jeopardize the secure fastening of the rods on and their electrical contact with the holding tongs. If the lateral pressure of the rods which are received by the inner springs of the membrane holder is too high, i.e., the centering effect is no longer sufficient to center the rods, the membranes are pushed off-center while the rods are entering the shielding facility, thus preventing the membranes from being damaged. In so doing, the accurate shielding of the field lines is nevertheless made possible. By contrast, in the shielding membranes of the art which are rigidly fastened to the shielding facility, the membranes made of elastic material gape upon lateral pressure exerted by the rods so that the shielding action is impaired at this place.
Within the lower membrane carrier and underneath the membrane, there are mounted, acting as inner centering springs, either several wings of centering springs which are tapering toward the center and are resiliently attached to the membrane holder or at least three resilient centrically arranged spring rods that constitute a guide, said inner centering springs having a centering effect on the rods by gripping the rods threaded through the membrane, thus acting as a centering guide. More specifically, the spring bars may be given an elongated shape and may, starting from a plane formed by the membrane holder and running slantways, form together with the free ends, an opening which is aligned with the center of the membrane holder, and through which the rods can be passed.
At least two non-rotatable spring bars attached at one end to the membrane holder and having mating non-rotatable centering guides attached to the free end of the membrane holder may be utilized as inner centering springs within the lower membrane carrier. Said spring bars are oriented in such a way that a rod, which may be threaded through the membrane held by the membrane holder, may be gripped by the spring bars. The guides provided for the rods are each prismatic and essentially parallel to the spring bars.
The inner centering springs accommodated within the upper membrane carrier may specifically be of an elongated shape and may, standing essentially upright on a plane constituted by the membrane holder, form, together with the free ends which are bent at an angle of at least 45xc2x00, preferably of at least 90xc2x0 and in particular of at least 180xc2x0, an opening which is aligned with the center of the membrane holder and through which the rods can be passed.
Upon exiting the rods out of the shielding device of the invention, the outer springs on the membrane holders ensure that the membranes center themselves again for the next batch of rods entering the device, i.e., that they return to their original position.
In order to adjust the device for the processing of various rod lengths or of various geometrical shapes of the rod areas to be plated and not to be plated, height adjusting facilities are provided by means of which the membrane carriers with the membrane holders and the membranes may be moved in vertical direction.
The device permits to conduct a method for partially electrochemically processing the rods in dip plants involving the following stages:
a. Grasping the essentially vertically oriented rods by means of appropriate holding elements;
b. Immersing the rods into a plating tank and into a membrane holder arranged within at least one membrane carrier, the plating tank being provided with tubular electrodes and with at least tubular one membrane carrier into which the rods may be at least partially entered;
c. Conveying the objects to be plated through one central opening each in at least one membrane held by the membrane holders.
In the process of passing through the at least one membrane, such rods which are not vertically arranged and the central openings in the membranes are automatically aligned.